SPR is an oscillation of a plasma of free electrons which exists, generally but not exclusively, at a metal surface boundary. SPR results in a resonant transfer of energy from photons, incident to the metal surface, to the plasma at the metal surface boundary. The quasiparticle resulting from the quantization of such an oscillation is a plasmon.
Typically, a SPR sensor comprises a prism and a metal film atop thereof. The surface of the metal film opposite to the prism forms an interface with an ambient medium. Typically, the metal film is made of gold or silver. An electromagnetic radiation beam, such as p-polarized light, is propagated through the prism and is totally internally reflected on the metal film, as a result of the metal film's having a refractive index lower than that of the prism. When some photons of the electromagnetic radiation beam incident to the prism-metal film interface resonantly transfer their energy to the electron plasma at the metal film-ambient medium interface, SPR occurs.
Such a transfer of energy is made possible through an evanescent wave partially penetrating into the metal. The evanescent wave is produced by the electromagnetic radiation propagating through the prism and incident to the metal film. In this manner, the electromagnetic radiation looses part of its energy to the evanescent wave and thus produces a coupling between a mode of the electromagnetic radiation and a mode of the surface plasmons.
SPR can be detected through a dip in an angular dependence of the intensity of the reflected electromagnetic radiation beam, or through a dip in a spectral intensity of the reflected electromagnetic radiation beam. In turn, position of the dip depends on the refractive index of the ambient medium. As a result, a change of the refractive index of the ambient medium will produce a shift of the dip.
After a SPR sensor has been calibrated for the ambient medium, it can be used to detect bio-molecules and other bio-materials, which tend to attach to some metal films such as gold and silver films. Upon attachment to the metal film, the bio-molecules or other bio-materials will produce a shift of the dip, thus enabling their being detected.
Propagating at the metal/dielectric interface, surface plasmons [1] are extremely sensitive to changes in the refractive index of the dielectric. This feature constitutes the core of many Surface Plasmon Resonance (SPR) sensors. Typically, SPR sensors are implemented in a Kretschmann-Raether prism geometry to propagate p-polarized light through a glass prism and reflect it from a thin metal film made, for example, of gold or silver and deposited on a facet of the prism [2]. The presence of a prism allows phase matching of an incident electromagnetic wave with a plasmonic wave at the metal/ambient dielectric interface at a specific combination of the angle of incidence and wavelength. Mathematically, phase matching condition is expressed as an equality between a plasmon wavevector and a projection of a wavevector of the incident electromagnetic wave along the metal/ambient dielectric interface. Since plasmon excitation condition depends resonantly on the value of the refractive index of the ambient medium within 200-300 nm from the metal/ambient dielectric interface, this method enables, for example, detection with unprecedented sensitivity of biological binding events on the metal surface [3]. The course of a biological reaction can then be followed by monitoring angular [3,4], spectral [5] and/or phase [6, 7] characteristics of the reflected light. However, the high cost and large size of commercially available systems [8] makes them useful only in laboratories, while many important fields and other applications remain out of the applicability of the method.
In order to miniaturize SPR biosensors, several waveguide-based implementations have been developed [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. In these sensors, light is launched into a waveguide core and then coupling of a guided mode with a plasmonic mode is used to probe for the changes in the ambient environment. To excite efficiently a surface plasmon the phase matching condition between the plasmon and waveguide modes has to be satisfied, which mathematically amounts to equality between their modal propagation constants. Ideally, one would use a single mode waveguide (SMW) with all the power traveling in a single Gaussian-like core mode operating near the point of resonant excitation of a plasmon (FIG. 1a). Near such a point most of the energy launched into the waveguide core could be efficiently transferred into a plasmon mode. Such an approach based on planar waveguides has been indeed demonstrated in the visible spectral region to provide several compact designs of SPR biosensors [9, 10, 11, 12, 13, 14]. However, for such single-mode, low index-contrast waveguides, the SPR coupling is realized at essentially grazing angles of modal incidence on the metal layer. As follows from the basic SPR theory [1], coupling at such grazing incidence angles leads to an inevitable decrease of sensitivity of a SPR sensor. Moreover, due to limitation to the lowest attainable value of the refractive index of waveguide materials, such sensors were demonstrated mostly in the visible spectral region where phase matching condition is easiest to reach. In principle, to increase the angle of modal incidence on the metal layer, high index contrast waveguides could be employed. However, as shown in FIG. 1a, quick inspection of a corresponding band diagram shows that phase matching between plasmon mode and a fundamental waveguide mode is not easy to realize. This is due to the fact that an effective refractive index of such a mode is close to the refractive index of the material forming the core of the waveguide, which is typically larger than 1.45 due to material limitations. The refractive index of a plasmon is close to the refractive index n of the ambient medium which is typically air, wherein n=1, and water, wherein n=1.3. Thus, large discrepancy in the refractive indices makes it hard to achieve phase matching between the two modes, with an exception with higher frequencies (λ<650 nm) where the plasmon dispersion relation deviates substantially from that of an analyte material.
Another solution to the phase matching and incidence angle problem is a coupling to a plasmon via the high order modes of a multimoded waveguide [15, 16, 17, 18, 19] (MMW). As can be seen from the plot of their dispersion relations (FIG. 1b), such modes can have significantly lower effective refractive indices than a waveguide core index. In such a set-up light has to be launched into the waveguide as to excite high order modes some of which will be phase matched with a plasmon mode. As only a fraction of higher order modes are phase matched to a plasmon, then only a fraction of total launched power will be coupled to the plasmon.